1. Field of the Invention
The present invention relates generally to a radio relay system, a radio relay apparatus, and a radio relay method, and particularly to a radio relay system for establishing radio communication with a transmitting/receiving apparatus through one or more radio relay apparatuses.
2. Description of the Related Art
In establishing radio communication, when the distance between a transmitting apparatus and a receiving apparatus is long, or when an obstacle lies between a transmitting apparatus and a receiving apparatus, for example, sufficient reception power may not be secured. As a result, the communications quality may be significantly degraded. In response to such a problem, a so-called radio relay transmission method (radio relay method) is conventionally used for conducting signal transmission in a broadcasting system or a mobile communication system. According to the conventional radio relay method, a radio relay apparatus (relay unit) receives a signal transmitted from a transmitting apparatus, amplifies this signal, and transmits the amplified signal.
FIG. 1 is a diagram showing a configuration of a radio relay system in which the conventional radio relay method may be implemented.
The radio relay system of FIG. 1 includes a transmitting apparatus 610, a radio relay apparatus 620, and a receiving apparatus 630. In the present system, the receiving apparatus 630 may receive a signal directly from the transmitting apparatus 610, or it may receive a signal from the transmitting apparatus 610 via the radio relay apparatus 620. The radio relay apparatus 620 is arranged to receive a signal from the transmitting apparatus 610 and transmit this signal to the receiving apparatus 630.
FIG. 2 is a diagram showing an exemplary configuration of the radio relay apparatus 620. As is shown in this drawing, the radio relay apparatus 620 may include a receiving antenna 701, an amplifier 702, and a transmitting antenna 703. A signal received at the receiving antenna 701 may be amplified at the amplifier 702 to at least a certain level, after which the amplified signal may be transmitted from the transmitting antenna 703 to the receiving apparatus 630 (see FIG. 1).
In Japanese Laid-Open Patent Publication No. 9-284195, a radio relay booster is disclosed that is capable of canceling a sneak path interference signal with high accuracy in a radio relay system as is described above. According to the disclosed technology, by repeating a process of changing the current canceller control variable consisting of attenuation characteristic information and phase rotation information by a small amount, detecting the residual interference signal, and updating the canceller variable in a direction for decreasing the residual interference signal, the sneak path interference signal may be cancelled with high accuracy.
In the radio relay system as is described above that uses a direct relay method, when a direct wave is present that is to be received by a receiving apparatus directly from a transmitting apparatus without being relayed by a radio relay apparatus, unnecessary fading may occur. Thereby, the application of the conventional radio relay method may not always lead to improvement in the communications quality; in fact, in some cases, a degradation of communications quality may occur as a result of applying the conventional radio relay method. This is described in detail below.
In the radio relay system of FIG. 1, given that the transmission line for the signal being transmitted is referred to as ‘channel’, the channel that is received by the receiving apparatus directly from the transmitting apparatus is denoted as hd, the channel that is received by the receiving apparatus from the transmitting apparatus via the radio relay apparatus is denoted as hi, and the channels hd and hi may be represented by the following formulas:hd=aeiθhi=a′eiθ′(a and a′ representing the amplitude, and eiθ and eiθ′ representing the phase).
Accordingly, the total channel htotal from the transmitting apparatus to the receiving apparatus including the radio relay apparatus may be represented by the following formula:htotal=hi+hd=aeiθ+a′eiθ′
In general, the condition, θ≠θ′, applies, and thereby, the received electrical field amplitude may not always be larger compared to a case in which only a direct wave is transmitted and received. For example, in an extreme case where a=a′ and θ=θ′+π, the phase may be inverted so that the channel signals cancel out each other. Thereby, the signal power received at the receiving apparatus may be very small, resulting in a significant degradation of communications quality compared to a case in which radio relay is not conducted.
Such a problem may also arise due to the occurrence of unnecessary fading in a case where signals are simultaneously transmitted using the same frequency band, or in a case where radio relay is conducted by plural radio relay apparatuses, for example.
It is noted that the conventional radio relay system is primarily used for terrestrial broadcasting in which the positions of the transmitting apparatus, the receiving apparatus, and the radio relay apparatus may be prearranged. In this case, the problem as is described above may not be such a great concern. However, this problem may have to be addressed in a case where the radio relay system is applied to a mobile communication system, in which the radio relay apparatus may be under the same environmental condition as a mobile terminal (rather than being implemented in a steel tower or a building); that is, the radio relay apparatus is implemented within a mobile terminal.
In recent years and continuing, with the proliferation of broadband technology in the communication field, high speed transmission is being demanded in mobile communication systems. In turn, attention is being directed to the MIMO (Multiple-Input Multiple-Output) system as a transmission scheme for increasing transmission speed using a limited frequency band.
However, in MIMO channel signal transmission where a signal received at the radio relay apparatus is directly amplified and transmitted, when a direct wave is present or when plural radio relay apparatuses are present, for example, the channel formed between the transmitting apparatus and the receiving apparatus may not necessarily correspond to a channel suitable for increasing the communications capacity for the MIMO communication. In other words, when there is high correlation between the channels formed by the transmitting and receiving antennas of the transmitting apparatus and the receiving apparatus, an increase in communications capacity may not necessarily be achieved through MIMO channel signal transmission.